November 26, 2012

Busy Beavers

By Dave Erler, Senior Naturalist

The leaves have changed their colors and now lie on the ground. Birds that head south for warmer climates are well on their way. But beavers are at their peak of activity as they too prepare for winter. From late October through November beavers are busy refurbishing an existing lodge or constructing an entirely new one. The beavers I watch seem to rarely use the same lodge for more than a year or two before building a new one or doing major reconstruction on an old one. I suspect the abundance of easily accessible woody building materials might be the deciding factor in whether to rebuild or relocate the lodge. When a new lodge is constructed the beavers start by piling up rocks, wood and mud. Once the pile is large enough they excavate the underwater entry tunnels and above waterline living chamber. One of the marvels of their construction is how haphazard it looks, but somehow they allow for a ventilation shaft from the living chamber through to the roof. During the winter it is relatively easy to locate the ventilation shaft as the warm air from inside the lodge melts an opening. On a cold winter day if you climb up on top of the lodge you can look down into the small shaft and see some amazing frost crystal formations resulting from the warm inside air meeting cold outside air.

Two colonies I have kept close tabs on have shown a great deal of variation where they build their lodges. Even though the dams they build have remained in the same place their lodges have alternated between being built up against the shoreline and out in the middle of the pond much like an island. Over the past six years a total of 10 lodges have been built and occupied. Of those ten 5 were built on the shore line and 5 as “island” lodges. I am at a loss as to why they located the lodges where they did. Water depth and distance from the dam seems to be random but obviously the beavers have a good reason to go through all that work. Both colonies have also attempted to dig out bank dens but they seemed to abandon those efforts perhaps due the same difficulty I have digging in New Hampshire without hitting some major boulders.

You might wonder why wait so late in the fall to get their house in order. Some speculate that doing their lodge construction or putting a refurbishing coat of mud on their current lodge late in the fall allows the fresh mud to freeze more solidly there by insuring predators have little chance of digging through to their living chamber. Beavers remain active all winter long although once the pond freezes you see little evidence of their activity. That’s mostly due to the fact that in addition to lodge preparation they are also busy cutting and anchoring a cache of fresh branches into the bottom mud just outside their lodges. This branch cache looks like a sunken brush pile and is sometimes mistaken as beginnings of an addition to their lodge. The cache of branches becomes a major source of food through the winter. The cold water keeps the branches fresh and edible right through until spring thaw if it isn’t consumed. Since the entrances to their lodge are underwater they can slip out and recover a branch to gnaw on when they are hungry without any obvious sign of activity from the world above the frozen pond. Although for the most part I see little beaver activity through the winter there are exceptions. If the winter is mild enough or we get a good midwinter thaw beavers will use the opportunity to come out on top of the ice to gnaw on a branch or aquatic vegetation they up rooted from the bottom of the pond.

If you’re looking for a fun late fall/early winter adventure visiting an active beaver pond is a fun thing to do. Just be careful going out on the ice until it is safe. And remember the ice near the dam is always thin and even the water near the lodge is often the last part of the pond to freeze. If you visit before freeze up and you’re there early morning or late afternoon there is a good chance you might catch the busy beaver in action.